1. Field of Art
This invention relates to spark ignition diagnostics, and more particularly to versatile ignition defeat and sensing of signals indicative of the firing time and dwell thereof.
2. Description of the Prior Art
In the aforementioned copending applications, there is disclosed and claimed apparatus for sensing low coil signals and high coil signals respectively related to the grounded and ungrounded end of the primary of the ignition coil of a spark ignition engine, to provide a signal which accurately delineates, without noise, the firing time and dwell period of the ignition cycles of the engine. One of the features of the apparatus disclosed and claimed in the aforementioned copending application is the ability to provide a conditioned signal delineating firing time from the dwell period, whether the related engine is being operated in an ignition defeated mode or in a full ignition mode. Many modern engines are provided with electronic switching modules in place of the previously-conventional, cam-operated breaker points shunted with a condenser, for the purpose of interrupting current through the coil primary, thus to induce the high voltage spark. Some of these systems include a protective resistance in series with the low coil signal line, whereas other such systems do not. In the past, ignition defeat has typically been made by connecting resistance, on the order of five ohms, in parallel across the coil primary when ignition defeat is desired. This is sufficient to limit the primary voltage to an extent that the high voltage of the coil secondary will be sufficient to cause an igniting spark in the spark plugs, yet, permit sensing the timing of the operation of the ignition system by monitoring signals on the high coil and/or low coil lines. But when there is a resistance (typically on the order of seven ohms) in series with the low coil line, the use of a typical ignition defeat would provide a total of twelve ohms across the primary, which allows more than the desired voltage to build up, so that ignition is sporadically observed while defeat is desired. The systems having seven ohms in series with the low coil line can be defeated by simply short circuiting high coil to low coil without doing any damage. But the systems without that resistance cannot be shorted from high coil to low coil without short circuiting the ungrounded end of the battery directly through either the breaker points or the electric switching module, which can cause great damage; and, naturally, there would be no signals to sense so as to delineate and measure the dwell and firing times. Thus, two different types of ignition defeat are required if various types of the newer and more conventional systems are to be accommodated for electronic sensing of timing of the ignition system while cranking with the ignition defeated. However, this requires an analysis of the particular type of engine under test and selectively providing either a dead short between high coil and low coil for those systems which have a series resistance built in or providing a shunting resistance between high coil and low coil for those systems which do not have a resistance built in. This hampers the operability of diagnostic systems which should be connectable to vehicles for diagnosis without undue modifications as a function of the particular type of engine under test.